Blood is commonly described as a complex red fluid consisting of plasma with a suspension of blood cells, mainly red blood cells. In an adult the blood volume is approximately 5 liters, of which 40-50% is red blood cells. The ratio between red cells and plasma is called hematocrit. Plasma consists mainly of water with proteins, sugar, vitamins, hormones, etc. A red blood cell, or erythrocyte, consists of a thin membrane as a kind of balloon in which there is water and a high concentration of the protein hemoglobin (Hb) in various forms. Hemoglobin is a complex molecule containing heme groups with iron atoms that can bind to and release oxygen and carbon dioxide in the circulation. Normally there is approximately 13-15 g hemoglobin per 100 ml blood in an adult, corresponding to 4-5 million red blood cells. Even a very small sample of blood taken from one person is a representative amount of blood cells for making measurements to determine certain properties and conditions of the blood. The hemoglobin value is a representation of the oxygen transportation capacity of the blood to other tissue and a parameter for patient diagnosis. Hemoglobin value is furthermore a primary safety and quality parameter in blood banking where blood is stored for transfusion purposes and collected as raw material for blood based industrial purposes.
Hemoglobin measurement is one of the most common diagnostic tests in the world today. Recently, development has taken place in the field of hemoglobin measurement, in that whole blood can be used as measurement material, thereby relaxing the need for hemolysation. An example of such an improved hemoglobin measuring device is disclosed in the International patent application number WO 2006/104451.
In addition to determining hemoglobin in a blood sample, other blood tests are routinely employed in primary care diagnosis. An example of such additional test is bleeding time tests. This test measures the time it takes for small blood vessels to close off and bleeding to stop and is performed to asses the platelet function of a patient. The Ivy method is the traditional procedure for testing bleeding time. The test involves inflating a sphygmomanometer to 40 mm Hg around the upper arm of the patient. A 5 mm deep incision is made with a special tool on the flexor surface of the forearm and the time is measured to cessation of bleeding. Every 30 seconds filter paper is used to draw off the blood.
The Ivy test is a rather unreliable test as several external factors can affect the test results, such as thickness of patient skin, ambient temperature, pressure applied when cutting skin, etc. In addition, the Ivy test can primarily only be utilized for detecting whether there are some problems with the platelet function in patient but is of little use for quantifying the platelet function of a patient.
WO 2004/109277 discloses an apparatus for determining the coagulation status in a blood or plasma sample. Magnetic particles added to the blood or plasma sample are caused to move through the sample by application of a magnetic field. Magnetic field sensors register changes in the response of the magnetic particles to the applied magnetic field and the coagulation status of the sample can be determined.
WO 2004/093641 discloses an acoustic blood analyzer with a transducer section of acoustic biosensors for measurement of blood coagulation in a sample. Acousto-mechanical sensors generate acoustic waves that penetrate the blood sample. As the blood coagulates on the sensors their mechanical properties change, resulting in measurable changes in the sensor natural frequency and power attenuation.